The present invention relates in general to a product that is used to reduce and preferably also diagnose electromagnetic emissions as particularly occur from electronic data prccessing equipment. More particularly, the invention is concerned with the use of ferrite-based materials for the reduction and diagnosis of emissions problems. Furthermore, in accordance with the present invention a substitute for ferrite-based materials is also derived.
Various government agencies have enacted regulations limiting the amount of emissions allowed from electronic data processing equipment. These regulations generally govern radiated signals between 30 and 1000 MHz. In the United States such regulations appear under FCC Rules, Part 15, Subpart J.
Electronic data processing equipment usually communicates to peripheral equipment via cables. The equipment contains devices such as clock oscillators, microprocessors, multiplexers and buffers, which produce a multiplicity of signals, such as square wave and sharp edge pulses. Generally, signals which are both high frequency (500 KHz or greater) and periodic tend to produce harmonics which can be detected by a nearby receiver, causing interference with licensed broadcast reception. Generally, the present state of the art limits the fastest high frequency, periodic signals generated by electronic data processing equipment to 20-100 MHz. The fundamental or harmonics of these signals causes the emissions which the rules are aimed at containing.
Generally, however, only the first few harmonics of these high frequency periodic signals have sufficient energy to cause emission problems. For most electronic data processing equipment in use today, emission problems are generally confined to frequencies below 300 MHz. Since the wavelength, at this frequency, is approximately one meter, the computers themselves are often not the primary source of the emissions. Although they contain high frequency periodic signals, they are usually not large enough to act as antennas. The primary "antennas" giving rise to radiated emissions are cables attached to the electronic data processing equipment.
The emissions are generally caused as follows: High frequency periodic signals, which are often due to the CPU clock, build up noise on inadequate ground rails (conductors). DC traces have inductance and therefore finite impedances at radio frequencies. RF currents passing through these inductances build up RF voltages. Devices using ground rails in common with clocks have these RF voltages incrementally coupled to their outputs. Therefore, cables attached to the equipment under test radiate with these clock derived RF energies.
I/O cables consist of a multiplicity of wires, each intentionally carrying its own signal. Radiated emissions, however, are primarily due to "common mode" noise, that is, the same unwanted RF noise appears on all the I/O leads. It is this unwanted RF noise which is the primary cause of emissions from data processing equipment. To a distant source, the cable is indistinguishable from a single wire, since all the wires have the same RF noise. Note that the digital signal which is intentionally sent down the cable is really differential in nature--return currents pass back down the ground wire in the cable.
Various techniques have been proposed for reducing these emissions, including bypassing (using decoupling capacitors) and shielded cables. Both of these schemes are described hereinafter in the detailed discussion. Both of these techniques have drawbacks. Bypass capacitors have inherent inductance limiting their effectiveness. Also, they must be carefully mounted so that their lead inductance is reduced. Shielded cables must have carefully designed backshells and be mounted directly against a metal chassis; otherwise, their effectiveness is limited. For these reasons, it has been necessary to develop other means for reducing emissions from I/O cables.
Accordingly, it is an object of the present invention to provide an improved means and system for reducing emissions from electronic data processing equipment and in particular from I/O cables associated therewith.